There are a number of industries wherein equipment is required to operate in an atmosphere of sulphur hexafluoride (SF6) gas. One particular industry is the electrical supply industry. SF6 has very low electric conductivity characteristics which lends itself as being a highly suitable insulating gas for high voltage equipment, for example, electric circuit breakers and transformers. Most equipment manufacturers specify that the equipment must be brought into service using new SF6. Industry standards dictate the level and type of impurities which may be present with SF6 to satisfy the definition of usable SF6.
New SF6 is relatively expensive. Furthermore, SF6 has serious environmental concerns with respect to its potential contribution to the greenhouse effect. These environmental concerns add to the cost of handling SF6 and minimising the release of SF6 to the atmosphere.
The normal handling of SF6 periodically occurs through the life of high voltage equipment. The SF6 gas must be removed from the equipment before the equipment can be serviced, repaired or decommissioned. Such handling can introduce additional contaminants to the SF6, for example, air, oil and water. Therefore, the removed SF6 may fall outside the standard for usable SF6 and will often prevent it from being re-used. Other contaminants may be present in the removed SF6 due to normal use in the equipment, for example, CF4 can be formed by arcing. The International Electrotechnical Commission IEC 60480, “Guide to the checking and treatment of sulphur hexafluoride (SF6) taken from electrical equipment”, published October 2004, provides information relating to the likely origin of various types of impurity, as follows:
TABLE 1Origin of SF6 impuritiesSF6 SITUA-POSSIBLETION & USESOURCE OF IMPURITIESIMPURITIESDuring hand-Leaks and incompleteAir, Oil, H2Oling and inevacuation;service DesorptionInsulatingPartial discharges: CoronaHF, SO2, SOF2, SOF4,function onlyand sparkingSO2F4In switchingSwitching arc erosionH2O, HF, SO2, SOF2,equipmentSOF4, SO2F2, CuF2, SF4,WO3, CF4, AIF3Mechanical erosionMetal dusts, particlesInternal arcMelting & decomposition ofAir, H2O, HE, SO2,materialsSOF2, SOF4, SO2F2,SF4, CF4,Metal dusts, particlesAIF3, FeF3, WO3, CuF2
IEC 60480 further provides recommended maximum contamination levels that can be present in re-usable SF6, as follows:
TABLE 2Maximum acceptable impurity levelsMAXIMUM ACCEPTABLE LEVELSRated absolute pressureRated absolute pressureIMPURITY<200 kPa>200 kPaAir and/or CF43% Volume3% VolumeH2O95 mg/kg25 mg/kgMineral Oil10 mg/kgTotal reactive50 μl/l total or 12 μl/l for (SO2 + SOF2) orgaseous decompo-25 μl/l HFsition products
In gas leak situations, due to the environmental issues, leaked gas is increasingly being contained until it can be removed from containment. However, SF6 is open to substantial contamination during containment. Therefore, SF6 removed from containment may fall way outside the standard for usable SF6.
Known SF6 gas recovery equipment normally utilises high pressure pumps and large storage vessels. The volume of freshly pumped gas into a containment vessel is relatively large. To be effective, the pumps must have a high capacity. However, high capacity pumps of the kind suitable for handling SF6 are cumbersome and expensive equipment. Such pumps must be protected from chemical attack by some of the decomposition products of SF6 and the gas stream must be filtered and purified before the gas enters the pumps. Furthermore, SF6 gas recovery tends to take a substantial amount of time which is undesirable, especially in gas leak situations.
U.S. Pat. No. 4,274,851, by the present inventor, discloses an apparatus for recovering gas from high voltage equipment using cryogenic pumping. The cryogenic pumping was found to be an efficient alternative to high capacity pump equipment. However, the disclosure does not teach how to produce a sufficiently pure form of SF6 for re-use.
It is an object of the present invention to provide a system and method for recovering a re-usable grade of SF6, said re-usable grade of SF6 being of a purity standard satisfying IEC 60480. It is a further object to provide a means of rapidly extracting SF6 from electrical equipment, for example, using facilities that are not sensitive to chemical attack by SF6 decomposition products and that allow subsequent purification at a later time and place. It is a further object to provide SF6 recovery without the need for high capacity pump equipment.